This SRFI cannot be supported by all Schemes. Therefore, by using it
you are restricting the portability of your code.

Isn't this true, in principle, of all the SRFIs?

No and yes.
It is certainly true that not all SRFIs *are* implemented in all
Schemes. However, most SRFIs *can* be implemented in all R5RS-compatible
Schemes, often trivially. This SRFI (and possibly certain others)
*cannot*.
The following two examples illustrate the difference:
If I write code that uses SRFI-1 and then discover that I want or need
to run the code in a Scheme that doesn't implement SRFI-1, I can
simply load Olin's reference implementation.
If I write code that uses SRFI-55 and then discover that I want or
need to run the code in Scheme48, I am hosed.

If we restrict ourselves to SRFIs that can be implemented on
any R5RS system without changes, we would get nowhere.
cheers,
felix